Another reason is that even the believers have a natural resistence to giving up meat, and they fear it will damage their movement. They want to keep it quiet, and for some strange reason, their opponents go along with keeping it quiet, probably because they think the less said about it the better. Big mistake IMO, especially if meat is in fact the Achilles Heel of the environmentalists. The American people are used to being scolded about oil, but if they're asked to give up meat, they'll begin to wise up, and start asking basic questions. It's this "leave well enough alone" mindset which prevents people from getting to the truth.
Finally, there's a natural inclination to think of oil as the culprit, not just because Big Oil is so widely demonized, but because we've all been conditioned from childhood to think of smokestacks and tailpipes as pouring out evil, filthy pollution. Never mind that we emit carbons and that they're organic. Oil companies are "bad." Farmers are "good."
Thus, it is counterintuitive to see meat as the problem. Frankly, I don't think man's oil consumption or meat consumption emits enough carbon to change the climate. But I believe in being fair."